Open the Contacts app on your iPhone
Tap a contact to open their details
Tap Edit
Scroll down and tap Delete Contact
Confirm by tapping Delete Contact
To delete multiple contacts: open Settings
Tap Contacts
Tap Accounts
Select the account that contains the contacts
Turn off Contacts sync for that account
To remove contacts from a specific account permanently: go to Settings
Tap Contacts
Tap Accounts
Select the account
Tap Delete Account (if available)
Confirm deletion
To delete all contacts on the iPhone: open Settings
Tap Contacts
Tap Accounts
Turn off Contacts sync for all accounts
Go to Settings
Tap General
Tap Transfer or Reset iPhone
Tap Reset
Tap Reset All Settings
After reset, turn Contacts sync back on for only the accounts you want